Leak tight vessels comprising a fiber reinforced material as their wall structure and methods for producing them are known in the art.
With “leak-tight vessel” is meant a substantially liquid-tight vessel or a substantially gas-tight vessel or both, wherein the permeability of the vessel for the liquid and/or gas to be stored inside the vessel is below a maximum prescribed limit for the given application the vessel is intended for. For example, in case the application is a hot water boiler application, the relevant permeability is the permeability of hot water under the intended storage conditions (e.g. temperature, pressure).
With “gas and/or liquid tight” is meant that it can be gas tight, or liquid tight, or both, depending on the intended application.
A known method for making leak-tight vessels, in particular pressure vessels, uses filament winding of continuous fibers impregnated with a thermoset resin over an inner bottle (also called “liner”) that will remain in the vessel after the filament winding step. The inner bottle is sufficiently rigid to be tightly overwrapped with continuous fibers, and is quite thick (e.g. 1-4 cm) to act as the gas and/or liquid barrier. A disadvantage of such a method is that the bottle (liner) is heavy and expensive.
Various methods for producing leak-tight vessels are known in the art. In one of these methods continuous fibers impregnated with a thermoset resin are filament wound over a plastic inner bottle (also called “liner”) that will remain in the leak-tight vessel after the production. Because during filament winding of continuous fibers a large pressure is exerted upon the object being wound, the plastic bottle needs to be sufficiently thick (e.g. >1 cm thick for a diameter of about 50 cm). At the same time, such a bottle also acts as the gas and/or liquid barrier for the leak-tight vessel, while the fibers wound around the bottle act as a protection layer. When producing pressure vessels, the bottle is usually made of a thermoplastic material, in order to avoid cracks due to the internal pressure. While such a bottle can provide a high barrier for the gas and/or liquid, it is heavy and expensive.
U.S. Pat. No. 4,760,949 describes a composite container for storage of products at non-atmospheric conditions. The composite container has a high barrier liner layer including a metal layer of vacuum deposited aluminum parallel with and spaced from the longitudinal edge of a synthetic plastic base thereby to define a first web that is helically wound around a cylinder in edge overlapping relation such that one longitudinal edge of the metal strip overlaps the other longitudinal edge of the metal strip by a given constant distance (d). The overlapping edges of the first web are hermetically joined by a heat-sealable bond between an adhesive layer covering the metal strip and the adjacent face of the first web, and a compatible heat sealable layer on the opposite face of the web. The method applies filament winding around a cylinder. After the fibrous cylindrical wall is removed from the mandrel, metal end parts are added to form a leak-tight vessel, and an end sealing compound is provided between the composite wall and the metal end part so as to obtain a hermetical connection.
A disadvantage of this method is that such a leak-tight vessel is not suitable to withstand high pressure (e.g. 2 bar or more). U.S. Pat. No. 3,367,815 entitled method and apparatus for forming filament wound vessels, patented Feb. 6, 1968, describes a method for forming a filament wound vessel comprising following steps:                Forming a first wound shell of resin impregnated filaments about a collapsible mandrel having a detachable fitting;        Curing the first shell to bond the fitting to the shell;        Removing the mandrel;        Mounting the fitting and shell on a shaft;        Forming a second wound shell of resin impregnated filaments amount the first shell while using the latter as a mandrel;        Curing the vessel. (abstract)        
The method described above is meant to solve the problems related to the use of an inflatable mandrel (column 1, lines 50-55), or a meltable or soluble mandrel (column 2, line 2).
Another prior art solution was the use of a ‘lost’ mandrel, left behind in the finished vessel (column 2, line 25).
A specific characteristic of this disclosure is that the partially wound shell, resulting from the above two first process steps, serves as a forming mandrel during the subsequent process steps. This procedure is said to permit removal of the relatively bulky mandrel components from the interior of the shell prior to the time that the latter is completely formed (column 2, lines 60-66).
Differently phrased, and as set out in column 3, lines 5-7, the mandrel components are removed from the interior of the vessel during the manufacture process thereof. This essential feature of the invention so disclosed is repeated in other terms again in column 3, lines 25-30 and column 4, lines 66-70, column 8 line 6 and again on line 19, column 10, lines 44-46, column 12, lines 40-41.
At least two disadvantages are imminently related to the process as disclosed in this prior art document.
The first is that although use is made of a reusable mandrel, such mandrel is not used during the entire manufacturing process. It is quite clear that from a mechanical point of view a reusable mandrel is a complicated and hence expensive piece of equipment. Also given this mechanical complication, its assembly and disassembly requires costly production time, and therefore using such piece of equipment during only a part of the manufacturing process of a fibrous vessel is in itself an inefficient operation.
Secondly, as the first wound shell resulting from the first two steps of this manufacturing process is used as a mandrel during the next manufacturing steps, such wound shell must exhibit a sufficient rigidity to effectively serve as mandrel during the subsequent winding process. Hence the thickness of the shell layer wound in the first two process steps must be thick enough to yield such rigidity. As a result the fibrous vessel produced according to this method is rather heavy weight, and the benefits of the expensive reusable mandrel used in this process are only partly used, thereby rendering this manufacturing method and the resulting fibrous vessels rather expensive.
GB Patent 1 255 738 entitled ‘A flexible and collapsible container and method of making the same’, complete specification published Dec. 1, 1971, describes a method for forming a yarn wound container comprising the following steps:                Preparing a metal former including separate component elements and annular clamps;        Applying a first coating of a thermoplastic synthetic resin;        Heat treating the coated former;        Winding on to the coated former a plurality of layers of flexible yarn;        Applying a second coating of thermoplastic resin to form an outer layer;        Heating the outer and inner coating layers so as to integrate the said coatings;        Removing the former by dividing it into its individual elements.        
(Page 1, line 31 until line 58)
Page 2, lines 113-117 teach heating the metal former to a selected temperature whereupon a thermoplastic synthetic resin is applied to its surface to form a coating.
The wall of the container so formed is constituted by windings of flexible yarn and an inner and outer layer of a pliable thermoplastic synthetic resin. Therefore the walls of the container itself are flexible and the container is collapsible. (page 2, lines 53-60).
A container manufactured according to this manufacturing process however clearly is not gas and/or liquid tight, as nowhere during the manufacturing process any measure or precaution in this respect has been made, nor have the materials that are used in this manufacturing process been selected with this aim in mind.
Also this manufacturing process implies, apart from the assembly and final disassembly of the former or mandrel, at least five subsequent steps, two coating application steps, one winding step and two heating steps. The overall manufacturing process hence is quite lengthy, and the resulting manufacturing cost of the containers consequently is high.
U.S. Pat. No. 3,334,780 entitled Pressure fluid container, patented Aug. 8, 1967, describes improved pressure fluid container constructions which preclude build-up of excessive internal pressures.
More specifically, the object of this disclosure is the provision of a pressure fluid container which, although fluid-impervious at normal temperature conditions, becomes fluid-pervious upon subjection to ambient temperatures above a known critical level. Thereby controlled escape of the fluid contents of the container and thus control of and limiting of pressure rise within the container is permitted (column 2, lines 6-11).
To this end, filaments are wound about a mandrel, such filaments being either moistened or impregnated with a suitable binder such as epoxy resin before, during or after the winding operation. After such a shell of predetermined thickness is formed, the resin is cured (column 4, lines 9-16). As a final step, the mandrel is removed through a large polar opening (column 4, line 20).
In carrying out the above, provision is made for closing the polar opening by means of a porous or fluid pervious material (column 4, lines 26-28). Also, the outer shell which envelops the inner shell, is porous, at least in the region surrounding the polar opening (column 4, lines 38-40).
The solution proposed in this disclosure however is quite difficult to realize in practical manufacturing conditions.
U.S. Pat. No. 3,508,677 entitled vessel for storing high-pressure gases, patented Apr. 28, 1970, discloses a vessel for storing gases under extreme pressure conditions as required e.g. in military aircraft.
Such vessel comprises three different layers: an inner layer comprising a thermoplastic resin liner; an intermediate diaphragm bonded to the inner liner; an exterior housing composed of various layers of resin impregnated fiber glass strands.
The manufacturing method for this type of vessel is based on the use of a water soluble mandrel (column 2, line 71). Given the specific application field for the vessels manufactured according to the method of this patent (military aircraft, aircraft fuselage), manufacturing cost is not the top priority. Key here is the ability for the vessel to store gases under extremely high pressures ((Column 1, line 26). Hence a one-time soluble mandrel can be used, but it is quite clear that the use of such one time-use mandrels for most commercial applications is not affordable. Also the inner liner consists of pre-formed laminated resin sheet sections that are joined to form an impermeable inner liner, again adding to the overall cost of the vessels manufactured according to this disclosure (FIG. 3).
Finally the inner layer of this type of vessels remains flexible so that it is capable of expanding and contracting when under pressure arises and/or pressure is released during use. Such inner flexibility however often gives rise to damage to the inner layer, whereby the vessel loses its leak-tight characteristic. For one-time use vessels as disclosed in the application field of this invention, this is not a concern, but for most commercial applications, the vessel should retain its gas and/or liquid tight property during extended periods of use.